Safety circuit for electrostatic spray gun

ABSTRACT

An improved high voltage sensing and disabling circuit having a desensitizing feature to compensate for initial turn-on of an electrostatic spray gun is disclosed. The circuit provides a safeguard against excessive electrostatic spray gun ionizing currents, but automatically compensates for current surges caused by line charging during turn-on.

United States Patent [191 Parson [111 3,809,955 [451 May 7,1974

[5 SAFETY CIRCUIT FOR ELECTROSTATIC SPRAY GUN [75] Inventor: Ronald F. Parson, St. Paul, Minn.

[73] Assignee: Graco lnc., Minneapolis, Minn.

[22] Filed: Mar. 26, 1973 [21] Appl. No.: 344,685

[52] US. Cl 317/3, 317/31, 321/14 [51] Int. Cl. H02h 3/08, HOZh 7/12 [58] Field of Search 317/3, 262 A, 31,321/11, 321/14; 118/4, 8,12

[56] References Cited UNITED STATES PATENTS 2,767,359 10/1956 Larsen et a1. 317/3 3,544,844 12/1970 Pellegrino 317/31 3,699,388 10/1972 Ukai 3,725,738 4/1973 Sokolsky 317/3 Primary Examiner-L. T. Hix I 1 Attorney, Agent, or Firm-Paul L. Sjoquist [5 7] ABSTRACT An improved highvoltage sensing and disabling circuit having a desensitizing feature to compensate for initial turn-on of an electrostatic spray gun is disclosed. The circuit provides a safeguard against excessive electrostatic spray. gun ionizing currents, but automatically compensates for current surges caused by line charging during turn-on.

17 Claims, 6 Drawing Figures FIG 1.,

GUN TRIGGER FIG, 2, k v /5 l2 7 4 CONTROL 4 POWER 1 OUTPUT CIRCUIT v SOURCE TO sun 22 SENSING cmcun' T PATENTEDMAY 119M 33,809,955

SHEET 2 OF 3 (53 5 55 l QYII PATENTEBMAY 7 197 $809,955

sum 3 [1F 3 BACKGROUND OF THE INVENTION .1. Field of the Invention This invention relates to electrostatic coating systems and more particularly to a safety apparatus for safeguarding against excessive ionizing currents which otherwise could occur in the operation and use of electrostatic coating systems. The invention is adaptable for use with either automatic electrostatic spray coating systems or with manual, hand-held electrostatic spray guns. In automatic spray coating systems, the spray apparatus is fixedly mounted relative to a moving conveyor,.wherein articles to be sprayed are attached to the moving conveyor and moved past the spraying apparatus. In manual systems, the spray gun is held in an operator's hand and manipulated so as to coat an article which is usually fixedly mounted. Since excessive ionizing currents are more likely to occur in a handmanipulated spray gun system, the invention is particularly useful in this type of manual system. In either type of system it is common to utilize electrical ionizing voltages approaching 100,000 volts between the spray gun electrode and the article to be coated. In order to obtain an efficient operation, it is desirable to place the spray gun electrode in reasonable proximity, 6-12 inches from the article to be coated, which is held at ground potential. However, if the spray gun electrode is brought too close to the article the electrostatic ionizing current increases rapidly and the possibility exists for creating a dangerous electrostatic spark. The present invention relates to a safety apparatus for sensing an increase in the electrostatic ionizing current and disabling the high voltage before an electrostatic spark is created.

2. Description of the Prior Art Sensing circuits which monitor the electrostatic ionizing current in electrostatic coating systems are known in the art. One example of such a circuit can be found in US. Pat. No. 2,509,277, issued May 30, 1950, wherein a resistance in series with a high voltage transformer secondary winding is described as a means for monitoring the electrostatic ionizing current. This patent describes a circuit for de-energizing the high voltage transformer when the ionizing current through the resistance exceeds the predetermined value. To compensate for surges in ionizing current which occur during initial turn-on of the apparatus, a manual disconnect feature is disclosed to disable the aforementioned safety circuit. Another example of the prior art can be found in Walberg U.S. Pat. No. 3,641,971, issued Feb. 15, 1972. This patent describes a more sophisticated safety circuit, but also uses manual means for overriding the operation of the safety circuit during initial turn-on.'

The present invention seeks to improve over this and other prior art attempts at safeguarding against excessive ionizing currents while allowing for initial current surges required during system turn-on, by providing a single circuit which automatically monitors ionizing currents and disables the high voltage when a maximum threshold ionizing current is reached, but adjusts the relative ionizing current threshold to compensate for current surges during system turn-on.

The invention also provides improved electronic apparatus toaccomplish functions heretofore requiring electro-mechanical switching .devices.

SUMMARY OF THE INVENTION This invention utilizes a resistance in series with a high voltage generating circuit for providing a voltage proportional to ionizing current. A threshold switching device is connected to this resistance for providing a switched output responsive to a predetermined voltage level representative of an excessive ionizing current. The threshold switching device is connected to a load cut-off device connected in a manner to remove the high voltage potential from the electrostatic spray gun electrode; and a desensitizing circuit is connected between the high voltage turn-on switch and the threshold switching device to provide an alternative threshold switching level during the period of initial turn-on of the electrostatic system. This desensitizing circuit is adapted to cause the threshold switching device to react only to ionizing currents that are significantly higher than currents required to initially charge cable capacitance, and after initial cable charging is accomplished the desensitizing circuit causes the threshold switching device to revert to normal ionizing current responsiveness.

It is therefore a principal object of this invention to provide an improved safety circuit which not only safeguards against excessive ionizing current during the operation of an electrostatic spray coating system, but also safeguards against excessive ionizing currents at a second level during the initial system turn-on.

An additional object of this invention is to provide a safety apparatus having improved electronic. components and avoiding electromechanical devices.

A further object 'of this invention is to provide an electronic safety circuit for use with electrostatic spray coating systems wherein the level of safe ionizing current can be pre-selected and adjusted to enable the use of a circuit with a wide variety of electrostatic spray coating'system elements.

Further objects and advantages will become apparent from the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a pictorial representation of an electrostatic spray coating system;

FIG. 2 is a block diagram of the principal electronic elements associated with said spray coating systems;

FIG. 3 is a detail schematic diagram of one of the electronic elements of FIG. 2;

FIG. 4 is a detail schematic diagram of another of the 7 electronic elements of FIG. 2;

FIG. 5 is an alternative and preferred embodiment of the circuit of FIG-.4;

FIG. 6 is yet another embodiment of the circuit of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates an electrostatic coating system. An electrostatic spray gun 3 is connected by means of suitable hoses to an electrostatic powder'supply hopper 1. The powder supply hopper 1 contains coating powder which is supplied under pressure to the spray gun 3 for application upon an article (not shown) to be coated. Electrostatic spray gun 3 has an electrical connection to high voltage power supply and associated circuitry 2 by means of high voltage cable 4. High voltage cable 3 I 4 may exist in a wide variety of lengths, typically rangingf'rom several feet up to 100 feet. Because of the length and electrical characteristics of high voltage cable 4, it has a considerable distributed capacitance which must be initially charged wheneverthe high voltage power supply is turned on. The charging of this cable imposes a significantly higher current requirement upon the high voltage power supply than is required during normal spray coatingoperations. A second cable 5 contains a wire connecting the trigger 7 of the electrostatic spray gun 3 to a control circuit within power supply 2 which will hereinafter be described. Trigger 7 on spray-gun 3 controls the high v'oltage generating circuitry within power supply 2. FIG. 2 illustrates a bloclcdiagram of the essential electronic elements relating to this invention. Control circuit 10 is activatedby means of trigger cable 5 to energize high voltage power source 15. Power source includes a transformer and rectifier circuit, so that a D.C.-voltage of magnitude SOKV-lOOKV is placed on high voltage cable 4 and coupled to a high voltage electrode (not shown) in electrostatic spray gun 3. The ionizing current caused by this high voltage is monitored by sensing circuit which 'is connected to high voltage transformer 15 by means of wire 19 and terminal 21. Wire 19 is connected to one side of the rectified high voltage output circuit. Sensing circuit 20 receives a control signal from control circuit 10 via line 9 and terminal 22, which control signal represents the initial turn-on state of the electrostatic high voltage. Sensing circuit 20 also has an output terminal 23 which is connected to control circuit 10 via line 11; this output terminal conveys a signal representative of excessive ionizing current,and causes control circuit 10 to disable high voltage to the electrostatic spray gun.

Control circuit 10 is activated by trigger 7 on electrostatic spray gun'3, which is an electrical contact switch connecting a resistance 33 to ground iwhenthe trigger 7 is depressed. Since the other side of resistance 33 is connected to a positive voltagepower source, a current flows through resistance 33, causing transistor '35 to FIG. 3 illustrates in simplified schematic form the circuit details of control circuit 10 and power source 15. The primary winding of high voltage transformer 16 is capacitively .connected to a saturable core inverter transformer 17. This saturable core inverter transformer is commonly used in the art for developing high voltages, and typically has a D.C. output voltage of approximately 400 volts. The output voltage charges capacitor l8, and silicon-controlled rectifier(SCR) 31 is periodically fired to dump the capacitor charge into the primary winding'of high voltage transformer 16. SCR 31 is tired upon application of a voltage pulse to its control element 32, in a manner known in the prior art. This voltage is transformed and rectified via rectifier circuit 30 to create a high voltage of magnitude SOKV-lOOKV on high voltage cable 4, which is conveyed to the electrostatic spray gun electrode. Rectifier circuit 30 may be any ofa variety of commonly-known means for rectifying alternating currents, preferably of the voltage-doubler or t'ripler type. The electrostatic high voltage potential exists between power source wire 19 and output cable 4..Wire 19 is connected to ground potential through a series resistance as will be described in connection with FIG. 4 hereinafter. The high voltage delivered to cable 4 is therefore generated by periodically applying a pulse to SCR 3 1 control element 32; removing these periodic pulses from control element 32 causes the high voltage to cease. The signal which drives SCR control element 32 is generated by control circuit 10 of FIG. 3 as will now be described.

turn on and conduct heavily. This raises the potential on line 9, and also at the base of transistor 36, causing transistor 36 to turn on and also conduct current. The current conducted through transistor 36 is drawn through lamp 38, turning it on. Lamp 38 is mounted on the control panel of high voltage power supply 2 and serves to indicate thecondition high voltage on;

lamp 38 is of a type which illuminates upon application of currents of the order of I00 milliamps.

When transistor 36 turns on and conducts current, it

alsocauses transistor 40 to turn on. The current conducted through transistor 40 is'of the order of less than several milliamps and, although drawn through lamp 39, is insufficient in magnitude to illuminate lamp 39.

" Current through transistor 40 is driven into an RC network consisting of resistor 46 and capacitorfll. The RC time constant of these elements controls the length of time required for voltage to develop across capacitor 41. Since unijunction transistor 43 has its control element 42 connected to the junction between resistor 46 and capacitor 41, the control element 42 will respond to the voltage across capacitor 41. Unijunction transistor 43 is a device of the type sensitive to threshold voltages at its control element, and characteristically switches its output when a predetermined input voltage threshold is reached. This, operating characteristic causes the output of unijunction transistor 43 to. suddenly conduct current when the voltage across capacitor 4l rises to a predetermined level. When this occurs, a positive-going voltage pulse is transmitted via terminal 12 to control element 32 of SCR 31,- causing SCR 31 tofire. y, l

When unijunction transistor 43-conducts output cur rent a discharge path is provided for thecharge on capacitor 41,..through unijunction element 42 and resistor 45. This discharge path allows the voltage on capacitor non-conductive or off state. At this point capacitor 41 again begins charging and the cycle repeats itself. Thus, it-is apparent that the aforementioned circuit forms an oscillator for periodically applying voltage pulses to control element 32 of SCR 31.

When trigger 7 is released, transistor 35 ceases conduction and the voltage at line 9 and the base of transistor 36 proceeds in a negative-going direction. This causes transistor 36 to cut off, thereby cutting off current flow through la'mp '38, and also cutting off conduction of transistor 40. When transistor 40 cuts off, the oscillator circuit of unijunction transistor 42 also cuts off, and SCR firing pulses are no longer delivered to SCR control element 32.

In summary, control circuit 10 acts in response to trigger 7 to turn on high voltage on indicator lamp -38 and to repetitively fire SCR 3] to generate a high of the effects of sensing circuit 20. The operation of control circuit in conjunction with sensing circuit will now be described, in particular with reference to FIGS. 3 and 4.

FIG. 4 illustrates sensing circuit 20, and will be described in terms of its interaction with control circuit 10 as illustrated in FIG. 3. Line 9 connects the collector of transistor 35 to input terminal 22 of sensing circuit 20. When trigger 7 is depressed, the voltage on line 9 goes positive, and this positive-going voltage is connected to the base of transistor 48 via input terminal 22 to cause transistor 48 to turn on and conduct current. Transistor 48 remains conducting for a period of time determined by the RC time constant of resistors 49 and 50 and capacitor 51. This turn-on time is variable and can be selected by adjusting resistor 50; resistor 50 is typically set to a value dictated by the electrical characteristics and length of high voltage cable 4, because the desired result is to cause transistor 48 to remain on for the length of time required to charge high voltage cable 4 to its ultimate electrostatic ionization potential.

For the length of time that transistor 48 remains conducting it effectively shunts across resistances 55, 56 and 57, connecting junction point 54 to ground potential. Since control element 62 of unijunction transistor 63 is also connected to junction point 54, it is also shunted to ground to maintain unijunction transistor 63 in the non-conducting off state. While in this off state, the voltage at output terminal 23 is held at a minimum value. Output terminal 23 is connected to the control element 59 of SCR 60, illustrated in FIG. 3 within control circuit 10; this connection is made via line 11. Thus, the generation of a signal on output terminal 23 is prevented while transistor 48 remains conducting; because of this, SCR 60 is prevented from firing.

After a suitable time interval, determined by the setting of resistor 50, transistor 48 ceases conduction and the voltage at junction point 54 returns to a potential determined by the relative values of resistances55-58 and the current into terminal 21. In particular, resistance 58 is connected via terminal 21 and line 19 to one output from high voltage power source 15. Thus, there is developed across the resistor network comprised of resistors 55-58 a voltage determined by the current flow from high voltage power source 15; this current flow is the high voltage electrostatic ionization current conducted between the spray gun high voltage electrode and the grounded target to be coated. The currentpath can be traced from line 19 at the high voltage power source 15 output, through terminal 21, through resistors 58 and 55 in series, through resistors 56 and 57 in parallel, to ground illustrated in FIG. 4 as point 47. In the embodiment described herein, the high voltage delivered to the spray gun electrode is of negative polarity.

The voltage at junction point 54 is proportional to this ionization current and is connected to the control element 62 of unijunction transistor 63to control the conductivity of the unijunction transistor 63. As the ionization current increases, the voltage at junction point 54 also increases; at some threshold potential unijunction transistor 63 conducts current, causing the voltage at terminal 23 to trigger positive. This threshold voltage point can be selected by adjusting resistor 57. The positive-going voltage at terminal 23 is connected, via line 11, to SCR control element 59 which causes SCR 60 to fire and become conducting. When SCR 60 becomes conducting, current flows through lamp 39, causing it to illuminate. Lamp 39 is mounted on the high voltage power supply panel and is representative of the condition high voltage overload when illuminated. The firing of SCR also causes transistor 40 to cut off, shutting off the oscillator action of unijunction transistor 43 and its associated circuitry. This, in turn, causes the voltage pulses at terminal 12 to cease, preventing further firing of SCR 31.

SCR 60 conducts current through lamp 39 and on transistor 36 to ground. SCR 60 will remain conducting for so long as transistor 36 is held in the on state, which is determined by trigger 7. If trigger 7 is released, transistor 35, and hence transistor 36, is cut off to break the conduction path through lamp 39, SCR 60, and transistor 36. In summary, when the high voltage ionization current increases to a predetermined magnitude, a pulse is generated by unijunction transistor 63 which causes the firing of SCR 60. This, in turn, illuminates h'igh-voltage-overload lamp 39 and disables the high voltage generating source 15. High voltage cannot again be generated until trigger 7 is released, causing a disruption of the series current path through transistor 36. Once trigger 7 has been released it can again be depressed to restart the high voltage generation as hereinbefore described.

FIG. 5 illustrates an improved variation of sensing circuit 20 wherein the circuit is responsive to excessive ionization currents even during the time interval required for electrostatically charging cable 4. The circuit of FIG. 5 is similar to that of FIG. 4 in all respects except that a resistor has been connected between junction point 54 and the collector of transistor 48. It will be recalled that, in FIG. 4, transistor 48 is gated on during a time interval approximately equal to the time required to charge the distributed capacitance of cable 4. During this time interval, the current will be higher than that required for normal operation because of the current needed for charging cable capacitance. Therefore, transistor 48 is used to shunt across resistors 55, 56 and 57 to effectively disconnect the ionization sensing circuit which would otherwise respond to this excessive ionization current to disable the high voltage.

The circuit of FIG. 5 provides an improvement thereon, because resistor 65 in series with transistor 48 effectively acts as a desensitizing circuit rather than a disabling circuit. With transistor 48 turned on, resistor 65 is connected in parallel with the resistor network comprised of resistors 55, 56 and 57. For a given ionization current level, this reduces the voltage at junction point 54 but does not clamp it to ground potential. Therefore, the sensing circuit comprised of unijunction transistor 63 and associated components is still responsive to excessive values of ionization currents and can still disable high voltage generation when excessive ionization currents are generated. For example, during the period of time required for charging distributed capacitance in cable 4, usually less'than one-half second, the current demand on high voltage source 15 may approach 200 microamps. Oncecable charging has been completed and normal operation resumes, the ionization current levels for normal operation are of the order of 20-100 microamps. However, if the high voltage electrode on spray gun 3 is positioned close to a grounded source when trigger 7 is first depressed, the high voltage power source will be heavily overloaded and the circuit comprised of resistor 65 and transistor 48 in series will enable the sensing circuit to respond to this condition to disable further generation of high voltage.

FIG. 6 illustrates another embodiment of sensing circuit 20. In this embodiment, the sensing circuit has a desensitizing feature that provides for a cable-charging current that is some fixed multiple of the normal overload ionization current; the multiple is determined by the ratio of the sum of resistances 70 and 71 to 71 [(R70 R71 71]. In this circuit the threshold switching value of unijunction transistor 63 is normally determined by the voltage at junction point 54, which is proportional to ionization current levels. However, unijunction transistor 63 is operated at a reduced voltage because of the effect of the resistor divider formed by resistances 71 and 70. Resistors 70 and 71 cause a reducedvoltage at junction point 75 and therefore make unijunction transistor 63 responsive to a reduced triggering voltage at, junction point 54. Of course, resistances 58, 55, 56 and 57 are sized to accommodate the reduced threshold switching voltage requirements of unijunction transistor 63, so that threshold switching occurs at a predetermined ionization current level.

When spray gun trigger 7 is initially depressed, transistor 48 is caused to conduct as before to desensitize sensing circuit 20 for a time interval required for charging distributed cable capacitance. However, inthe operation of the circuit of FIG. 6, when transistor 48 becomes conductive, it applies the. full power supply voltage (+V) to junction point 75, thereby raising the threshold voltage sensitivity of unijunction. transistor 63. For example, if resistor 70 and 71 were chosen to normally provide a voltage at junction point 75 of +V/2, when transistor 48 is turned on the potential at junction point 75 suddenly becomes +V, raising the threshold voltage of unijunction transistor 63 by a factor of 2. This desensitizes sensing circuit 20 to require an ionization current of twice the normal overload ionization current before high voltage power source will be disabled. The circuit of FIG. 6 therefore provides for a cable charging surge current that is a fixed multiple of the normal ionization overload current.

What is claimed is:

l. A high voltage load sensing and disabling circuit having a desensitizing feature to compensate for initial turn-on, comprising:

' a. means for developing a first voltage proportional to current into said high voltage load, said means serially connected to said high voltage load;

b. a threshold switching device connected to said means for developing a first voltage, and adjustable to provide a switched output responsive to a preselected first voltage level;

c. a load cutoff device connected to said high voltage and to said threshold switching device, and responsive to said switched output to remove the high voltage on said load;

d. means connected to said load, for manually turning on said high voltage; and

e. a desensitizing circuit connected to said means for manually turning on said high voltage and to said means for developing a first voltage, said desensitizing circuit being activated for a predetermined time after manual turn-on of said high voltage to cause said threshold switching device to provide a switched output responsive to a different first voltage level.

2. Apparatus as claimed in claim 1 wherein said desensitizing circuit further comprises means for selecsensitizing circuit further comprises a resistance" switchable in parallel connection with at least a portion of said means for developing a first voltage.

4. Apparatus as claimed in claim 3 wherein said load cutoff device further comprises an oscillator circuit for delivering pulses at an output under normal conditions and wherein said pulses are stopped upon receipt of said threshold switching device switched output.

5. Apparatus as claimed in claim 4 wherein said load cutoff device further comprises a silicon-controlled rectifier (SCR) for receiving said pulses and for firing said SCR to cause generation of an alternating voltage, whereby the stopping of said pulses causes said SCR to stop firing.

6. A safety circuit for use with an electrostatic spray gun apparatus having a high voltage power source, a cable fortr'ansmitting high voltage to a spray gun electrode and a trigger for energizing'the power source, comprising: I i

a. means responsive to said-trigger for activating the high voltage power source;

b. current sensing means connected to said high voltage power source forsensing the magnitude of current associated with said high voltage;

0. a control circuit having an input connected to said current-sensing means and an output connected to said high voltage source, the control circuit generating a signal at its output in response to a presclected signal level at its input from said currentsensing means, said output signal causing deactivation of said high voltage power source;

d. a desensitizing circuit connected to said current sensing means and to said trigger to cause the control circuit output signal to be generated in response to a second and different control circuit input signal level, said desensitizing circuit becoming activated for a predetermined time interval after said trigger is engaged.

7. Apparatus as claimed in claim 6 wherein said desensitizing circuit further comprises a series-connected switching device and resistor coupled to said control circuit input, said switching device being closed in response to engaging said trigger.

8. Apparatus as claimed in claim 7 further comprising a resistance-capacitance circuit connected within said desensitizing circuit to cause said switching device to subsequently open upon-a predetermined accumulation of voltage charge on said capacitance.

9. Apparatus as claimed in claim 6 wherein said desensitizing circuit further comprises means for providing an offset voltage to, said control circuit input in an additive sense to said control circuit input signal level.

10. Apparatus as claimed in claim 8 further comprising a variable resistance in said resistance-capacitance circuit for selectively providing avariable time control over the subsequent opening of saidswitching device.

1 l. A safety circuit for disabling a high voltage power supply output on the occurrence of either of two overload current conditions, the first overload condition occurring immediately after the power supply is switched on and the second overload condition subsequently occurring, comprising:

a. overload current sensing means for generating a signal proportional to high voltage supply current, said overload current-sensing means serially connected at said high voltage supply output;

b. a threshold switching device having an input connected to said overload current-sensing means and having an output connected to the high voltage power supply disabling circuit;

0. a switching device having an output connected to said threshold switching device input and an input terminal, whereby at least a portion of said overload current sensing means is shunted by said switching device output;

d. a resistance-capacitance circuit connected between said switching device input terminal and said high voltage power supply switch to cause said switching device to become activated only during a time interval determined by the values of the resistance and capacitance.

12. Apparatus as claimed in claim 11, further comprising a resistance in series with said switching device output and said threshold switching device input.

13. Apparatus as claimed in claim 12 wherein the overload current-sensing means further comprises a variable resistance.

14. Apparatus as claimed in claim 13 wherein said threshold switching device further comprises a unijunction semiconductor device having its control element at the switching device input and including a pulsegenerating circuit at its output, said pulse-generating circuit responsive to a predetermined input signal level.

15. A safety circuit for disabling a high voltage power supply output on the occurrence of either of two overload current conditions, the first overload condition occurring immediately after the power supply is switched on and the second overload condition subsequently occurring, comprising:

a. overload current-sensing means for generating a signal proportional to high voltage supply current, said overload current-sensing means serially connected at said high voltage supply output;

b. a threshold switching device having an input connected to said overload current-sensing means and having outputs serially connected within a resistancedivider circuit;

c. a conductor connected at a first point in said resistance divider circuit and connected to the high voltage power supply disabling circuit;

d. a switching device having an output connected at a second point in said resistance divider circuit to shunt at least a portion of the resistance in said resistance divider circuit, and having an input terminal;

e. a resistance-capacitance circuit connected between said switching device input terminal and said high voltage power supply switch to cause said switching device to become activated only during a time interval determined by the values of the resistance and capacitance.

16. Apparatus as claimed in claim 15 wherein the overload current-sensing means further comprises a variable resistance.

17. Apparatus as claimed in claim 16 further comprising a shunt resistance between said second point in said resistance-divider circuit and circuit ground. 

1. A high voltage load sensing and disabling circuit having a desensitizing feature to compensate for initial turn-on, comprising: a. means for developing a first voltage proportional to current into said high voltage load, said means serially connected to said high voltage load; b. a threshold switching device connected to said means for developing a first voltage, and adjustable to provide a switched output responsive to a preselected first voltage level; c. a load cutoff device connected to said high voltage and to said threshold switching device, and responsive to said switched output to remove the high voltage on said load; d. means connected to said load, for manually turning on said high voltage; and e. a desensitizing circuit connected to said means for manually turning on said high voltage and to said means for developing a first voltage, said desensitizing circuit being activated for a predetermined time after manual turn-on of said high voltage to cause said threshold switching device to provide a switched output responsive to a different first voltage level.
 2. Apparatus as claimed in claim 1 wherein said desensitizing circuit further comprises means for selectively reducing said first voltage proportional to current into said high voltage load.
 3. Apparatus as claimed in claim 2 wherein said desensitizing circuit further comprises a resistance switchable in parallel connection with at least a portion of said means for developing a first voltage.
 4. Apparatus as claimed in claim 3 wherein said load cutoff device further comprises an oscillator circuit for delivering pulses at an output under normal conditions and wherein said pulses are stopped upon receipt of said threshold switching device switched output.
 5. Apparatus as claimed in claim 4 wherein said load cutoff device further comprises a silicon-controlled rectifier (SCR) for receiving said pulses and for firing said SCR to cause generation of an alternating voltage, whereby the stopping of said pulses causes said SCR to stop firing.
 6. A safety circuit for use with an electrostatic spray gun apparatus having a high voltage power source, a cable for transmitting high voltage to a spray gun electrode and a trigger for energizing the power source, comprising: a. means responsive to said trigger for activating the high voltage power source; b. current sensing means connected to said high voltage power source for sensing the magnitude of current associated with said high voltage; c. a control circuit having an input connected to said current-sensing means and an output connected to said high voltage source, the control circuit generating a signal at its output in response to a preselected signal level at its input from said current-sensing means, said output signal causing deactivation of said high voltage power source; d. a desensitizing circuit connected to said current sensing means and to said trigger to cause the control circuit output signal to be generated in response to a second and different control circuit input signal level, said desensitizing circuit becoming activated for a predetermined time interval after said trigger is engaged.
 7. Apparatus as claimed in claim 6 wherein said desensitizing circuit further comprises a series-connected switching device and resistor coupled to said control circuit input, said switching device being closed in response to engaging said trigger.
 8. Apparatus as claimed in claim 7 further comprising a resistance-capacitance circuit connected within said desensitizing circuit to cause said switching device to subsequently open upon a predetermined accumulation of voltage charge on said capacitance.
 9. Apparatus as claimed in claim 6 wherein said desensitizing circuit further comprises means for providing aN offset voltage to said control circuit input in an additive sense to said control circuit input signal level.
 10. Apparatus as claimed in claim 8 further comprising a variable resistance in said resistance-capacitance circuit for selectively providing a variable time control over the subsequent opening of said switching device.
 11. A safety circuit for disabling a high voltage power supply output on the occurrence of either of two overload current conditions, the first overload condition occurring immediately after the power supply is switched on and the second overload condition subsequently occurring, comprising: a. overload current sensing means for generating a signal proportional to high voltage supply current, said overload current-sensing means serially connected at said high voltage supply output; b. a threshold switching device having an input connected to said overload current-sensing means and having an output connected to the high voltage power supply disabling circuit; c. a switching device having an output connected to said threshold switching device input and an input terminal, whereby at least a portion of said overload current sensing means is shunted by said switching device output; d. a resistance-capacitance circuit connected between said switching device input terminal and said high voltage power supply switch to cause said switching device to become activated only during a time interval determined by the values of the resistance and capacitance.
 12. Apparatus as claimed in claim 11, further comprising a resistance in series with said switching device output and said threshold switching device input.
 13. Apparatus as claimed in claim 12 wherein the overload current-sensing means further comprises a variable resistance.
 14. Apparatus as claimed in claim 13 wherein said threshold switching device further comprises a unijunction semiconductor device having its control element at the switching device input and including a pulse-generating circuit at its output, said pulse-generating circuit responsive to a predetermined input signal level.
 15. A safety circuit for disabling a high voltage power supply output on the occurrence of either of two overload current conditions, the first overload condition occurring immediately after the power supply is switched on and the second overload condition subsequently occurring, comprising: a. overload current-sensing means for generating a signal proportional to high voltage supply current, said overload current-sensing means serially connected at said high voltage supply output; b. a threshold switching device having an input connected to said overload current-sensing means and having outputs serially connected within a resistance divider circuit; c. a conductor connected at a first point in said resistance divider circuit and connected to the high voltage power supply disabling circuit; d. a switching device having an output connected at a second point in said resistance divider circuit to shunt at least a portion of the resistance in said resistance divider circuit, and having an input terminal; e. a resistance-capacitance circuit connected between said switching device input terminal and said high voltage power supply switch to cause said switching device to become activated only during a time interval determined by the values of the resistance and capacitance.
 16. Apparatus as claimed in claim 15 wherein the overload current-sensing means further comprises a variable resistance.
 17. Apparatus as claimed in claim 16 further comprising a shunt resistance between said second point in said resistance-divider circuit and circuit ground. 